It depends on the kind of vulnerability. Say you have a vulnerability that allows writing to arbitrary pages, then an attacker on RWX system can write malicious code into pages that would get executed. In W^X environment, the attacker needs to find a W page and write to it before it becomes the X page.

This isn't a 100% mitigation, but it does make it harder to exploit.

JavaScript JIT been source of so many RCE vulnerabilities.